1. Field of the Invention
The present invention relates to a continuous method for manufacturing foam beads from polymers.
2. Discussion of the Background
Various methods of manufacturing foam beads are known. European EP-A-0 053 333 discloses dispersing polymer particles held together with a volatile expansion agent and a dispersant in water in a closed heated vessel. Once the dispersion is heated to a point above the softening point of the polymer, the polymer particles become impregnated with the expansion agent. Then, while maintaining the pressure in the vessel, one end of the vessel is opened and the contents are sprayed out into a low pressure atmosphere causing the polymer particles to expand into a foam. A similar method employing filler-containing polymer particles and dispensing with a volatile expansion agent is disclosed in German DE-OS 21 55 775. However, both of these methods are discontinuous and provide only relatively low production capacity because substantial time must be consumed in rinsing, filling, and heating. In addition, it is difficult in a discontinuous process to achieve a uniform product quality. Japanese Patent JP-A-59-067,022 describes a continuous method of manufacturing foam beads wherein thermal processing is carried out stagewise in six stirred reactors. A slurry comprising a polymer and dispersing medium is passed through each reactor and is de-pressurized in a low pressure space. One of the six reactors is required for adding the gaseous expansion agent and one is required for expelling the dispersion. The pressure in the dispersion expelling vessel is higher than in the vessels in which thermal processing is carried out. All of the vessels must be designed to be suitably pressure resistant, which results in high apparatus cost. The number of stirred heated vessels, pumps, and auxiliary equipment renders the system uneconomical from an equipment cost standpoint.
According to the state of the art, polymer particles are impregnated with a volatile expansion agent during thermal processing. Typical expansion agents include hydrocarbons such as propane, butane, pentane, hexane, and heptane, alicyclic hydrocarbons such as cyclopentane and cyclohexane, halogenated hydrocarbons such as trichlorofluoromethane, dichlorodifluoromethane, dichlorotetrafluoroethane, dichlorofluoromethane, trichlorotrifluoroethane, methyl chloride, ethyl chloride, and dichloromethane, and in certain cases inorganic gases such as carbon dioxide and nitrogen. These expansion agents are dissolved by the polymer particles under pressure, and are liberated into the low pressure space during expansion. It is desirable for economic, environmental, and safety reasons for the liberated expansion agent to be recovered and recycled after the expansion. However, this can be accomplished only at great expense for a discontinuous process as described in European EP-A-0 140 059. In European EP-A-0 140 059, a liquid-filled gasometer is used to store the liberated gas until the gas can be reused for a new reaction mixture. In the case of a high boiling expansion agent, the agent is condensed and stored for the interim in liquid form.
A common feature of all discontinuous processes for manufacturing foam beads is that as the dispersion is passed out of the pressure vessel, the pressure in the vessel is lowered by increasing the gas space in the vessel. Accordingly, there is a continuous decrease in the degree of expansion of the expelled particles. As described in European EP-A-0 095 109, this problem can be solved only by continuously adding expansion agent during the expansion so as to maintain the partial pressure of the expansion agent. Another feature which is important in establishing high quality foam is the use of nozzles having one or more openings, wherewith the openings have a specific diameter. European EP-A-0 234 320 requires the use of nozzles wherein the ratio of the length to opening diameter of the nozzle is in the range 4-100. Otherwise foam beads with nonuniform cell diameters and degrees of expansion are obtained.